1. Field of the Invention
The present invention relates to a method and apparatus for molding a metal, and particularly magnesium alloy or the like, using an injection-molding machine and a hot runner.
2. Description of the Related Art
In general, a thixo-molding apparatus has an injection mechanism and a clamping mechanism. Mold material, which is metallic chips of magnesium alloy or the like supplied from a hopper, is transferred by a screw into a cylinder heated to a predetermined temperature to be put into a liquidized or a semi-melted state, which develops thixotropy (the property exhibited by certain gels of becoming fluid when stirred or shaken and returning to the semisolid state upon standing, while not varying the temperature), in a co-existing state of a solid and a liquid, by applying a shearing force through the rotating screw. After this metallic material has been charged to a tip end of the cylinder, it is injected into a mold, held at a predetermined pressure and cooled to be a metallic molded product.
In this regard, a prior art system for molding magnesium alloy in this thixo-molding apparatus employs a molding cycle, shown in FIG. 3, comprising a mold-clamping process T1, a gate melting process T2, an injection-pressure increase (solidifying) process T3, a material-metering process T4, a mold-opening process T5, a product withdrawal process T6, a mold-lubricant coating process T7 and an air-blow process T8, while the solidifying process and the material-metering process are carried out in parallel to each other.
That is, first, the mold-clamping process for closing the mold is carried out. Then, the gate-melting process T2 for heating a hot runner is carried out to melt a plug (metallic material) at a tip end of a plug of the hot runner. When the injection becomes possible through the gate-melting process T2, the injection-pressure increase (solidifying) process T3 is carried out, wherein the melted or semi-melted metallic material is injected while increasing the pressure for preventing a sink mark from occurring due to the solidification shrinkage. Thereafter, the material-metering process T4 for preparing material for the next injection is carried out. During the metering of material, the metallic material filled in the mold is quenched by the heat-conduction to the mold and solidified. If the metallic material has been completely solidified and the mold product is in a state capable of being withdrawn from the mold, the mold-opening process T5 for opening the mold is carried out. Thereafter, the product withdrawal process T6, the mold-lubricant coating process T7 and the air-blow process T8 are conducted to complete one cycle of the molding processes. In the above cycle, as the material-metering process needs a relatively long time, it starts at an instant corresponding to the initiation of the solidifying process so that it is in time for the completion of the mold-clamping process.
However, when the material-metering process starts parallel to the solidifying process, the other processes must proceed after the material-metering process has completed, whereby a waiting time is necessary until the mold-clamping process has completed. In the thixo-molding method, it is an optimum condition, for obtaining a high-quality metallic mold product, that the metallic material is injected into the mold while exhibiting the thixotropy so that an apparent viscosity is lowered. However, as described before, if there is a waiting time after the completion of the material-metering process prior to the injection, the thixotropy of the metallic material in the co-existing state of solid and liquid charged in the cylinder is lost, and there may be a problem in that the metallic material is injected while maintaining a high viscosity to deteriorate the quality of the metallic mold product.
To solve this problem, Japanese Unexamined Patent Publication No. 2001-25852 proposes a method for molding a metallic product using a thixo-molding apparatus. In this method, a time at which the material-metering process starts is determined to make the completion of the mold-clamping process coincide with the finishing of the material-metering process, so that the metallic material as metered is immediately injectabled into the mold just clamped, whereby the above-mentioned waiting time is eliminated. However, this method is problematic in that the mold-lubricant coating process is carried out after a mold-opening process and prior to the mold-clamping process so that the metallic material-metering process and the mold-lubricant coating process are simultaneously carried out while the mold is in a open state. Thus, there is a risk in that molten metal drops down from the tip end of the injection nozzle to adhere to a touch surface of a nozzle or a metallic mold face to result in lack of touching of the nozzle or clamping of the mold, which may cause a risk of flash during the injection. Also, the mold-lubricant itself may splash out of the mold to worsen the working environment.
According to the metal molding method disclosed in the above publication, although the molding-lubricant coating process requiring a relatively large working time is carried out simultaneously with the material-metering process, the former is independent from processes other than the material-metering process, whereby it does not provide a solution for saving time necessary for the molding cycle. Thus, a saving of the cycle time cannot be expected because the effect of saving the solidification time by the use of a hot runner is reduced by the process requiring the relatively large working time.